Coal-fired power generation plants, municipal waste incinerators, and oil refinery plants generate huge amounts of flue gases that contain substantial varieties and quantities of environmental pollutants, such as sulfur oxides (SO2, and SO3), nitrogen oxides (NO, NO2), and heavy metals such as mercury (Hg) vapor.
The destructive effects of various coal-burning pollutants on human health and on the ecosystem were recognized a long time ago. For example, SOx and NOx have been linked to the outbreak of respiratory diseases in the affected areas. They also form acid rains, which damage forests, fisheries, and architectures. As for Hg, it is a potent toxin to the nervous system. Exposure to mercury can affect the brain, spinal cord, and other vital organs. It is particularly dangerous to developing fetuses and young children.
Mercury and other pollutants may be captured and removed from a flue gas stream by injection of a sorbent into the exhaust stream with subsequent collection in a particulate matter control device such as an electrostatic precipitator or a fabric filter. Adsorptive capture of Hg from flue gas is a complex process that involves many variables. These variables include the temperature and composition of the flue gas, the concentration and speciation of Hg in the exhaust stream, resonance time, and the physical and chemical characteristics of the sorbent.
It is well-known that activated carbon is particularly effective as a chemical adsorbent. Given this property, activated carbon is used as an adsorbent in a variety of industrial processes. For example, the EPA and various other entities have studied and advocated the use of activated carbon in the treatment of flue gases produced during the combustion of coal, such as the flue gases produced at coal fired power plants. This process, which is known as “activated carbon injection,” has been touted as a potentially effective means for reducing the mercury emissions that typically accompany the combustion of coal. One potential drawback to the use of this system is the tendency to increase the amount of fly ash waste produced.
Despite the touted benefits of activated carbon injection, the efficacy of the process is believed to be limited by some of the inherent characteristics of the activated carbon. For example, it is believed that the porous nature of the activated carbon, which is the very property that makes the activated carbon useful in the process, allows the product to absorb large amounts of the water vapor and/or unwanted condensation present in both the ambient atmosphere (such as water vapor absorbed during storage of the activated carbon) and the flue gas environment. Once this water vapor and/or unwanted condensation has been absorbed, the effective pore volume of the activated carbon (that is, the pore volume that is available for adsorption of mercury) can be dramatically reduced. And this reduction in effective pore volume means that each kilogram of activated carbon used in the process is less effective than it would otherwise be if the activated carbon had not absorbed the water vapor. This reduction in efficacy means the process is overall less efficient. Though powdered activated carbon (PAC) is somewhat effective to capture oxidized mercury species such as Hg2+, PAC is not as effective for elemental mercury, which constitutes a major Hg species in flue gas, especially for subbituminous coals and lignite. Therefore, there is a need to provide a filter system that can abate the heavy metals such as mercury for a low cost.